1. Field of the Invention
The present invention generally relates to stent coatings that include bioactive compounds that inhibit restenosis.
2. Description of the Related Art
Stents are often used in the treatment of atherosclerosis, a disease of the vascular system in which arteries become partially, and sometimes completely, occluded with substances that may include lipids, cholesterol, calcium, and various types of cells, such as smooth muscle cells and platelets. Atherosclerosis is a very common disease that can be fatal, and methods of preventing the accumulation of occluding compounds in arteries are being investigated.
Percutaneous transluminal angioplasty (PTA) is a commonly used procedure to break up and/or remove already formed deposits along arterial walls. PTA can also be used to treat vascular occlusions not associated with atherosclerosis. During PTA, a catheter is threaded through a patient's arteries until the occluded area to be treated is reached. A balloon attached to the end of the catheter is then inflated at the occluded site. The expanded balloon breaks up the mass of occluding substances, resulting in a more open arterial lumen. However, there is a risk that the artery may re-close within a period of from one day to approximately six months of the procedure. This re-closure is known as restenosis. Accordingly, a balloon-only angioplasty procedure often does not result in a permanently reopened artery. To prevent restenosis, scaffolding devices called stents are deployed in the lumen of the artery as a structural support to maintain the lumen in an open state. Unlike the balloon and the catheter used in an angioplasty procedure, the stent usually remains in the artery as a permanent prosthesis. Although technically feasible, removal of the stent from the artery is generally avoided.
Stents are typically elongated structures used to keep open lumens (e.g., openings in the body) found in various parts of the body so that the parts of the body containing those lumens may function properly. Stents are usually implanted at their site of use in the body by attaching them in a compressed state to a catheter that is directed through the body to the site of stent use. The stent can be expanded to a size which enables it to keep the lumen open by supporting the walls of the lumen once it is positioned at the desired site.
The lumens of blood vessels are common sites of stent deployment. Vascular stents are frequently used in blood vessels to open the vessel and provide improved blood flow. The stents are typically hollow, cylindrical structures made from struts or interconnected filaments. Vascular stents can be collapsed to reduce their diameter so that the stent can be guided through a patient's arteries or veins to reach the site of deployment. Stents are typically either coupled to the outside of the balloon for expansion by the expanding balloon or are self-expanding upon removal of a restraint such as a wire or sleeve maintaining the stent in its collapsed state.
The stent is allowed to expand at the desired site to a diameter large enough to keep the blood vessel open. Vascular stents are often made of metal to provide the strength necessary to support the occluded arterial walls. Two of the preferred metals are Nitinol alloys of nickel and titanium, and stainless steel. Other materials that can be used in fabricating stents are ceramics, polymers, and plastics. Stents may be coated with a substance, such as a biodegradable or biostable polymer, to improve the biocompatibility of the stent, making it less likely to cause an allergic or other immunological response in a patient. A coating substance may also add to the strength of the stent. Some known coating substances include organic acids, their derivatives, and synthetic polymers that are either biodegradable or biostable. Biostable coating substances do not degrade in the body, biodegradable coating substances can degrade in the body. A problem with known biodegradable and biostable stent coatings is that both types of coatings are susceptible to breaking and cracking during the temperature changes and expansion/contraction cycles experienced during stent formation and use.
Stents located within any lumen in the body may not always prevent partial or complete restenosis. In particular, stents do not always prevent the re-narrowing of an artery following PTA. In fact, the introduction and presence of the stent itself in the artery or vein can create regions of trauma such as, e.g., tears in the inner lining of the artery, called the endothelium. It is believed that such trauma can trigger migration of vascular smooth muscle cells, which are usually separated from the arterial lumen by the endothelium, into the arterial lumen, where they proliferate to create a mass of cells that may, in a matter of days or weeks, occlude the artery. Such re-occlusion, which is sometimes seen after PTA, is an example of restenosis. Coating a stent with a substance to make the surface of the stent smoother and to minimize damage to the endothelium has been one method used to create stents that are less likely to contribute to restenosis.
Currently, drug therapy for restenosis primarily consists of the systemic administration of drugs. However, delivering drugs in this manner may result in undesirable side effects in other areas of the body unrelated to the vascular occlusion. Also, the administered dose of a drug that is delivered systemically is less effective in achieving the desired effect in the local area of the body in which it is needed. For example, an anti-restenosis drug delivered systemically may be sequestered or metabolized by other parts of the body, resulting in only a small amount of the drug reaching the local area in which it is needed.
Stents with bioactive compounds or drugs in or on their coatings can be used. One class of drugs that can be used in stent coatings is restenosis inhibitors. There remains a need for coatings that can be shown to actually release the restenosis inhibiting compounds in their active forms. Further, there is a need for stents that can carry drugs and release them in a sufficient concentration to produce the desired effect. In particular, there is a need for such stents that can inhibit restenosis.